Whole genome duplications or polyploidy are frequent in biology but their physiological significance is poorly understood. Polyploidy occurs during development, cellular stress, tissue damage, cancer, and evolution. Although polyploidy likely confers some short-term advantages, it has costs. Most notably, in many situations newly formed polyploid cells appear to be genetically unstable. In this proposal we will take molecular genetic approaches to define the mechanisms leading to genetic instability in polyploid cells. In the last funding period we have developed several cell systems that uniquely position this laboratory to attack this question. We developed a breast cancer animal model that provided the first direct test of the longstanding hypothesis that passing through an unstable tetraploid intermediate can promote tumorigenesis. Using budding yeast, we discovered a genetic phenomenon called ploidy-specific lethality that provides an experimental avenue to define underlying mechanisms of genetic instability in polyploidy cells: certain genes are not required for viability in haploid or diploid cells but become essential in polyploidy cells. We conducted a genome-wide screen and found a small and specific subset of genes (39/3740 screened) that when compromised result in ploidy-specific lethality. Strikingly, almost all of these genes affect aspects of genetic stability. Based on these preliminary results, we will take complementary approaches in both yeast and mammalian cells to define how polyploidy affects genetic stability. We propose experiments with the following aims: Aim 1. How does tetraploidy promote whole chromosome aneuploidy in yeast? Aim 2. How does polyploidy affect recombination and the ability of cells to acquire growth-promoting mutations? Aim 3. What mechanisms lead to chromosomal aberrations in tetraploid mammalian cells?